scsi: sd_zbc: Fix variable type and bogus comment

Fix the description of sd_zbc_check_zone_size() to correctly explain that
the returned value is a number of device blocks, not bytes.  Additionally,
the 32 bits "ret" variable used in this function may truncate the 64 bits
zone_blocks variable value upon return. To fix this, change "ret" type to
s64.

Fixes: ccce20fc ("sd_zbc: Avoid that resetting a zone fails sporadically")
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: stable@kernel.org
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>